New: directories can have an index 'page'

I've decided that sometimes I really want a directory to have an index page, not just a list of the contents. So now I can, with the unimaginatively named 'index' view. It's mostly template based; the normal template uses inject::index to display an __index file. However, the view has several special properties:

• unlike other directory views, it is not inherited by subdirectories.
• it is only listed as an available view in the page tools area if there is a file called __index in the directory.
• if __index is a redirection, the index view will just generate a redirect to the target.

(Thus, the index view could be used to replace the wikiroot configuration directive.)

The index view is valid on directories even without a __index page in the directory. Right now the template just displays a normal directory listing in that case.